1. Field of the Invention
The present invention relates to an image reproducing apparatus, for example utilizing an electrophotographic process, and more particularly to an image forming apparatus in which a latent image is formed by scanning a photosensitive member with a laser beam or the like and is then rendered visible.
2. Description of the Prior Art
An example of such image reproducing apparatus is a laser recording apparatus as disclosed in the Japanese Patent Laid-open No. Sho 51-8949. This apparatus makes use of a reproducing process toner in which is deposited in the area exposed to the laser beam, or a process in which toner is deposited in the area not exposed to the laser beam. In either process it has been difficult to correctly record a fine line with a sufficient image density.
The laser beam scanning is usually conducted by deflecting a laser beam with a rotating polygonal mirror. The laser beam, after deflection generally has a diameter of 1 to 5 mm and is focused onto a photosensitive member through an f..theta. lens. The focal length of said lens must be selected large enough to expand the width of the image. On the other hand, the diameter of the laser beam incident on the rotating polygonal mirror is limited since the mechanism for this mirror cannot be made excessively large. Also it is known that the spot diameter focused on the photosensitive member is proportional to the focal length of the f..theta. lens and to the wavelength of the laser beam and is inversely proportional to the beam diameter entering the lens. For this reason, the minimum spot diameter obtained on the photosensitive member is practically limited.
Consequently the focused spots may mutually overlap between the adjacent scanning lines when they are positioned close to each other. Thus, in the process in which the non-image area is exposed to the laser beam, a fine line or a small dot, which should not be exposed, is in fact weakly exposed to the laser beam for the adjacent scanning line, leading to a weak and blurred latent image which produces unclear recording with insufficient density after image development. A similar situation occurs in the process of exposing the image area and depositing toner thereon, in which case a fine line or a small dot is not overlapped with the beam of the adjacent line and has therefore a lower exposure to light in comparison with a large image area exposed with overlapping lines. Therefore, a weak latent image results.